Due to their small size and low weight, flat-panel displays have become increasingly popular for volume-limited and portable applications. See "Flat-Panel Displays," Scientific American, March 1993, p. 90. Transmissive displays such as liquid-crystal (LCD) displays have been especially well received due to their extremely low power consumption and potential for high resolution, including full color applications.
However, as they are non-emissive, such transmissive displays require some form of backlighting. Conventional backlights for high-brightness LCD displays, such as those used in high-performance aircraft cockpits, typically use a serpentine or "grid" fluorescent lamp or an array of linear lamp segments. A conventional diffuser is ordinarily used between the lamp and the LCD in an attempt to transform the highly structured lamp illumination into unstructured, uniform illumination behind the LCD. Diffusely reflecting surfaces, such as flat white paint are often used behind the lamp(s) to direct as much of the light output as possible back toward the diffuser and the LCD.
Conventional diffusers are ordinarily formed by a process which produces a roughened surface or embedded scattering sites on or within a substrate material. Conventional transmissive filters, for instance, include flashed opal, sand-blasted glass and various flexible films exhibiting light-diffusing properties. In general, these approaches do not efficiently direct the light emitted by the backlight source toward the display panel. This is due to the fact that although light rays incident upon a conventional diffuser are scattered, the axis of the scattered light is usually substantially the same as the corresponding ray of incident light, which has a low probability of already being in the direction of the display panel.
Furthermore, even when the incident light and scattered light are in the general direction of the display panel, conventional diffusers are prone to backscatter and above which further limits their overall efficiency.
Generally speaking, efforts to improve the uniformity of conventional diffusers further degrade their efficiency. For example, by stacking conventional diffusers, or by using diffusers that scatter through a wide range of angles, uniformity will be improved, but backscatter and transmission loss will be experienced at each diffusing stage. Moreover, multiple panels increase manufacturing costs, or increase the depth of the backlight structure, or both.
It has been proposed to use holographic-type diffusers in conjunction with flat-panel displays such as LCDs, but for the purpose of facilitating readout when viewed at an angle oblique to the display surface, and not to improve the uniformity of backlighting. In these applications the diffusers are placed between the display and the observer as a projection screen, and not behind the display. In U.S. Pat. No. 5,037,166, for example, a holographic optical system is used to prevent unwanted reflections and glare as when the instrument panel is within an aircraft cockpit. In U.S. Pat. No. 5,046,793, a holographic diffuser is used to provide chromatic correction, or color balance, in addition to redirecting display information to one or more viewing locations oblique to the display surface.
Thus, there remains a need for a highly efficient diffuser, including a holographic-type diffuser, that could be employed for more efficient and uniform illumination of a flat-panel display such as a liquid-crystal (LCD) display. With a sufficiently high aspect ratio scattering pattern, the improved device would be capable of efficiently "filling in the gaps" of light between tubular-type sources of backlighting, thus providing a high-brightness display without light bands or patterns revealing the shape of the underlying sources.